This invention relates generally to electrodes for discharge lamps, and more specifically to an improved electrode which exhibits a tighter seal, improved electrode alignment and improved seal integrity through the reduction of cracks when sealed in a quartz envelope.
The present invention is directed to an improved electrode for a discharge lamp which exhibits superior stability and minimum cracking when sealed in the neck of a quartz glass envelope.
Sealing the shank portion of tungsten electrode in the neck of a quartz glass envelope results in stresses caused by differences in thermal expansion and contraction of the materials in contact, the quartz glass and the tungsten metal. There has always been a problem in the field with respect to cracking occurring in the envelope when the shank of the electrode is sealed in the neck portion.
With regard to addressing problems such as envelope cracking at the contact area with the shank portion of the electrode, the prior art appears to have taken a mechanical approach to addressing and solving the problem.
In U.S. Pat. No. 2,518,944 a foil is wrapped around the shank portion of an electrode to prevent the quartz from adhering to the electrode rod and enhance stability of the structure.
In U.S. Pat. No. 3,706,900, a metal helix is used to surround two straight ends of a filament body or electrode which is said to provide resistance to vibration and shocks.
U.S. Pat. No. 4,968,916 is directed to an improved lamp structure having an improved electrode structure. In this structure, coil filaments are situated in opposite neck portions of an envelope forming a light source so as to cause the electrodes to be axially aligned within the light source and keep the shank of the electrode from intimate contact with the envelope, thereby preventing the condensation of mercury and allowing for substantial vaporization of the metal halide ingredient at the neck portion. In addition, the coils function to prevent thermal expansion of the electrode from cracking the envelope.
It can be seen from the above teachings of the prior art, that a separate mechanical component such as a metal wrap or coil has long been used to enhance stability and/or reduce cracking in the neck portion of quartz glass envelopes.
There has, therefore, always been a need in the art for a method of accomplishing the above objectives without resorting to the use of an additional component within the lamp structure.
It is therefore an objective of the present invention to provide an electrode which exhibits superior stability and eliminates the cracking problems associated with sealing the electrode shank into the quartz envelope of a quartz discharge lamp.
It is a further object of the present invention to provide an electrode which exhibits minimal cracking when sealed within the neck of a quartz discharge lamp and which does not require the use of any added component to the lamp structure.
It is yet another object of the present invention to provide a specially treated electrode having resistance to cracking when sealed in a quartz glass envelope.
It is yet a further object of the present invention to provide a superior electrode which exhibits a specially treated shank portion which exhibits a tighter seal in a quartz glass envelope.
It is a further object of the present invention to provide a method for making an electrode which exhibits superior stability and minimal cracking when sealed in a quartz glass envelope.
It is yet another object of the present invention for making a tungsten electrode having a specially treated shank portion which exhibits a tighter seal and improved electrode alignment when sealed in a quartz glass envelope.
The present invention relates to a tungsten electrode which has a specially treated shank portion which exhibits a tighter seal and improved electrode alignment when sealed in a quartz glass envelope and reduces stress cracking within the seal neck of the envelope. More specifically, the electrode of the present invention contains a shank portion which has been specially treated to form a thin outer layer of elemental tungsten at the base portion of the shank which results in improved properties when sealed in a quartz glass envelope. The invention is also directed to a method of making a tungsten electrode suitable for use in a quartz discharge lamp which includes providing a tungsten electrode of a predetermined configuration having a tip portion and a shank portion. A substantially uniform oxide coating of tungsten is formed on a selected portion of the shank of the electrode. The oxide coating is then treated to reduce the oxide to substantially elemental tungsten which is in the form of a coherent thin layer loosely bonded over the selected shank portion. This thin outer elemental tungsten layer exhibits superior properties when sealed in a quartz envelope which results in a dramatic reduction in cracking in the neck portion of the envelope in the area adjacent the seal of the shank with the quartz glass in the neck portion. Further, this thin outer elemental tungsten layer allows for a substantially tighter seal with a significant reduction in the cracking in the neck portion of the envelope in the area adjacent the seal of the shank with the quartz glass in the neck portion.
In one embodiment, a tungsten oxide layer is formed on a predetermined, defined area of an electrode shank by exposing the area to an oxidizing atmosphere at a suitable elevated temperature for a time sufficient to build the oxide layer. The oxide layer is subsequently converted to an elemental tungsten layer by firing in a wet hydrogen furnace at a temperature of at least about 1200xc2x0 C. which results in the formation of a loosely bonded tungsten surface layer.
It is well known that the onset of rapid oxidation of tungsten will occur at temperatures above 500xc2x0 C. Oxides of tungsten in the form WO3, tungsten trioxide, yellow-green in color, and W2O5, tungsten hemipentoxide, blue in color, are formed in this process. In the process of the present invention the heating of the tungsten is considerably higher, typically at or about 1200xc2x0 C. At this temperature the initial onset of oxidation is rapid and the rate of reaction slows as the oxide layer thickness increases. In fact the rate of oxide formation appears to be inversely proportional to the oxide layer thickness. Therefore, time as well as temperature are two important factors in the development and control of the process. It is further known that tungsten must be heated above 700xc2x0 C. in a hydrogen reducing atmosphere for any practical reduction of tungsten oxides. In fact at temperatures below about 700xc2x0 C. tungsten oxides will persist and are characterized by visible color as is illustrated in Table 1.